1. Field of the Invention
This invention relates to a process for preparing polyurethane emulsions. More particularly, this invention relates to a process for preparing homogeneous, stable, nonionic, self-emulsifiable polyurethane emulsions.
2. Description of the Prior Art
Various processes for preparing polyurethane emulsions are known. For example, an emulsifier-free polyurethane emulsion, namely, a so-called self-emulsifiable polyurethane emulsion can be prepared according to a known process comprising forming an isocyanate-terminated urethane prepolymer from a water-soluble polyhydroxyl compound and an organic polyisocyanate employed in an amount in excess of the stoichiometric amount, chain-extending the thus-formed urethane prepolymer with an active hydrogen atom-containing low-molecular-weight chain extender and dispersing the resulting polyurethane composition in water. However, the water resistance of a polyurethane resin prepared from the thus-obtained polyurethane emulsion is very low.
According to another known process, a polyurethane emulsion is prepared by dispersing a polyurethane composition in the presence of an emulsifier.
In general, polyurethane resins have excellent physical and chemical properties. However, the physical and chemical properties of a polyurethane resin film formed from a polyurethane emulsion prepared according to known conventional processes, such as those mentioned above, are greatly inferior to those of a film formed of a cross-linked polyurethane resin, because it is difficult to introduce cross-linkages in emulsion-type polyurethane resins by chemical means or because the emulsifier remains in the polyurethane resin. This is a serious problem in the field of practical use of polyurethane emulsions.
If a polymer such as polyurethane has strong cross-linkages, the polymer is infusible and insoluble, and handling thereof is difficult or often impossible during the preparation step, and therefore, it is substantially impossible to prepare an emulsion of a cross-linked polyurethane by dispersing it in water. If such a polyurethane can be dispersed in water, the resulting polyurethane emulsion is generally quite unstable.
To improve the water resistance of a polyurethane resin, there has also been proposed a process in which a part of a water-soluble polyhydroxyl compound to be used for the preparation of a polyurethane emulsion is replaced by a water-insoluble polyhydroxyl compound. According to this process, however, it is difficult to introduce strong cross-linkages into the polyurethane resin of a polyurethane emulsion, and the physical and chemical properties of the resulting polyurethane resin are not satisfactory from the practical viewpoint.
As one effective means for imparting a cross-linking property to a polyurethane resin, there is known a process comprising reacting epichlorohydrin to the amino groups of a polyurethane-urea polyamine obtained by chain-extending a urethane prepolymer with a polyalkylene polyamine. However, since the reaction occurs in a polymer system the percentage of addition of epichlorohydrin is very low and it is impossible to impart a sufficient cross-linking property to a polyurethane resin by this known process.